Thin film Magnetoresistive Random Access Memory (MRAM), which includes a Magnetic Tunneling Junction (MTJ) cell, can be fabricated in a variety of memory cell embodiments. The MTJ cell essentially includes a pair of magnetic layers with an insulating layer sandwiched therebetween. One of the magnetic layers has a fixed magnetic vector and the other magnetic layer has a changeable magnetic vector that is stable when either aligned with or opposed to the fixed magnetic vector. When the magnetic vectors are aligned, the resistance of the MTJ cell, i.e. the resistance to current flow between the magnetic layers, is a minimum, Rmin, and when the magnetic vectors are opposed or misaligned the resistance of the MTJ cell is a maximum, Rmax. The value change of this resistance may be on the order of thirty percent. Therefore, for a low resistance value of 10K ohms, the high resistance value could be about 13K ohms. A sense amplifier for an MRAM needs to detect this difference in value. Since the nominal value of the resistance has variation due to processing, it is useful to detect the state of a bit by comparing the resistance of the tunnel junction in a bit to a nearby midpoint reference (the averages of the maximum and minimum conductances) that may be formed as a midpoint of a reference bit in the high resistance state and a reference bit in the low resistance state. It is also important to maintain symmetry to balance the loading from the parasitic resistance and capacitance of the bit lines and the column multiplexing.
U.S. Pat. No. 6,445,612, entitled “MRAM with Midpoint Generator Reference and Method for Readout” and assigned to the Assignee of the present invention, offers one approach to reading data stored in MTJ cells, wherein a reference column including a midpoint generator are positioned adjacent a data column. Memory cells of the data column and the midpoint generator include similar magnetoresistive memory elements (MTJ elements). The MTJ elements of the midpoint generator are each set to one of Rmax and Rmin and connected together to provide a net resistance at a point therebetween. A differential read-out circuit is coupled to the data column and the reference column for differentially comparing a data resistance to a reference resistance. The configuration of MTJ elements in the midpoint generator divides the voltage applied to the reference column such that each MTJ has a reduced voltage applied across the insulating layer compared to MTJ elements on the data column. Since MTJ elements are susceptible to time dependent dielectric breakdown (TDDB) of the insulating layer and TDDB has a strong acceleration with applied voltage, this reduction in voltage applied to the MTJ elements on the reference column increases the amount of time they can be biased for a given level of reliability. In this application, and others using MTJ elements to approximate a midpoint reference, the time that bias is applied to the midpoint reference can be many times more than the time applied to MTJ elements on data columns because the midpoint reference is used during many or all of memory accesses while each MTJ on data columns are typically biased during a small fraction of all memory accesses. Because of this, use of the MTJ configuration in the midpoint generator increases the overall reliability of an MRAM.
Most known sense amplifiers have output terminals that are very responsive to movement on the input nodes of the sense amplifier. This responsiveness causes swings on the output nodes as the input nodes are charged to their steady state levels. During swings in voltage, capacitive imbalance may dominate the transient signal, resulting in loss of differential signal and speed of operation.
U.S. Pat. No. 6,600,690, entitled “Sense Amplifier for a Memory having at least Two Distinct Resistance States” and assigned to the same Assignee as the present invention, provides a fast and efficient read operation of the memory bit cell, wherein a bias circuit applies voltages to a sense amplifier and the sense amplifier develops an average reference current from a reference high bit and a reference low bit with which a differential signal develops in comparison with a bit cell current. Within the sense amplifier, careful distribution of the capacitive load allows for equal capacitive loading on the bit and reference signals, thus optimizing the differential signal. Effective precharge and equalization incorporated into the sense amplifier minimizes the effect of parasitic capacitive imbalances and further improves the speed of operation. The bias circuit, which may be common to multiple sense amplifiers, must include a midpoint reference closely matched to the midpoint conductance of the reference bits connected to the sense amplifier to provide voltages to the sense amplifier that allow maximum speed of operation. Mismatch between the midpoint reference used in the bias circuit and the midpoint conductance of the reference bits connected to the sense amplifier will cause precharge to non-optimal levels and increase the common mode movement on the output terminals of the sense amplifier, causing speed degradation. Therefore, it is best for maximum speed of operation to be able to set the state of each MTJ element in the midpoint reference used in the bias circuit.
When the MTJ elements in an MRAM are “Toggle” MRAM elements, meaning that they are designed to be switched using the Toggle MRAM switching method described in U.S. Pat. No. 6,545,906, no reliable direct write method exists. To set the state of MTJ elements in a midpoint reference, a self referencing method, like that outlined in U.S. Pat. No. 6,760,266 is necessary. Therefore, in order to set the state of each MTJ element in the midpoint reference, it is preferred that each of the MTJ elements can be isolated and independently biased by the circuit performing the self referencing write sequence.
Accordingly, it is desirable to use an MTJ configuration in a midpoint reference for an MRAM sense amplifier bias circuit that has a reduced voltage applied across each MTJ element and in which each MTJ can be set to a desired state, which for “Toggle” MRAM requires that each MTJ element can be isolated for individual biasing. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.